Backward wave oscillator having an ion collecting probe at the downstream end



United States Patent ABSTRACT 6F THE DISCLOSURE Improved backward wave oscillator performance is obtained by incorporating an ion draining probe at the downstream end portion of the oscillator. The probe is disposed internally of a helix slow wave circuit and has its free end protruding preferably a few periodic lengths within the helix. This design permits the probe to be biased at relatively high negative voltages and even down to cathode potential without adversely affecting beam transmission or interfering with beam-wave interaction along the major axial extent of the slow wave circuit. The design is especially effective in providing a potential slope at the downstream end of the oscillator which effectively eliminates the shielding effect of the slow wave circuit in an oscillator employing complete beam interception on the helix.

This invention relates in general to microwave traveling wave electron discharge devices of the backward wave oscillator type and more particularly to such devices having improved means for reducing spurious and FM noise generation resulting from ion oscillations and the like.

Techniques for reducing ion oscillations in microwave traveling wave tubes of the backward wave oscillator type are set forth in US. Patent No. 2,991,391 by W. L. Beaver, issued July 4, 1961. In particular there is disclosed in said patent the utilization of a specially designed collector operating below anode or circuit potential which results in reduced spurious noise output through drainage of positive ions normally trapped within the beam along the axial extent of the slow wave circuit. With the advent of backward wave oscillator tubes which utilize beam current interception along the axial extent of the slow wave circuit in order to improve electronic efficiency as pointed out in detail in US. patent application Ser. No. 409,521, filed Nov. 6, 1964 by Richard H. Ohtomo and assigned to the same assignee as the present invention, the utilization of a negatively biased probe type collector such as set forth in the aforementioned William L. Beaver application, has been found somewhat inadequate in providing significant positive ion drainage.

The present invention is particularly directed to overcoming the defects in the prior art ion draining techniques as set forth in the aforementioned William L. Beaver application through the utilization of a probe negatively biased with respect to the slow wave circuit and disposed partially co-extensive with and internally of the slow wave circuit along the axial extent thereof at the downstream end portion thereof. The utilization of an axially disposed positive ion draining probe which protrudes within the slow Wave circuit a few periods and which is negatively biased with respect to the circuit has been found to result in reduction of spurious noise output of as much as 30 db in comparison with the aforementioned positive ion draining probe techniques which are external to the downstream termination of the slow wave circuit as set forth in the William L. Beaver application. The ineffectiveness of the prior art ion draining probe as set forth in the aforementioned William L. Beaver application with regard to slow wave circuits employing beam current interception along the axial extent of the slow wave circuit has been determined by theoretical and experimental analysis to be a result of the shielding effect of the slow wave circuit itself.

The present invention further teaches: the utilization of high tensile strength high melting point refractory metals or alloys for the material of the probe itself in order to eliminate any possibility of melting of the probe if the electron beam should strike it, such as for example during exhaust operations when the probe is at helix potential. Furthermore, the selection of a high tensile strength refractory material will minimize vibration and sag of the probe during use in a backward wave oscillator which is used in a vibration environment. The material of the probe should also be selected with the criterion of minimizing poisoning of the cathode through evolution of gaseous products during use. Such refractory metals as for example, tungsten, molybdenum, tantalum and rhenium and alloys thereof are, per the teachings of the present invention, eminently suitable in this respect.

It is therefore an object of the present invention to provide a microwave traveling wave electron discharge device of the backward wave oscillator type with improved means for draining positive ions trapped along the axial extent of the slow wave circuit within the electron beam.

A feature of the present invention is the provision of a microwave linear beam backward wave oscillator with a positive ion draining probe disposed partially co-extensive with and internally of the slow wave circuit of the oscillator at the downstream end portion thereof and in DC. isolation with respect to the circuit.

Another feature of the present invention is the provision of a linear beam microwave backward wave oscillator having a slow wave circuit disposed along the axial extent with a probe extending internally of the slow wave circuit at the downstream end portion thereof and biased at least 50 volts or greater negative with respect to the circuit voltage.

Another feature of the present invention is the provision of a microwave linear beam backward. wave oscillator with a positive ion draining probe disposed partially coextensive with and internally of the slow wave circuit of the oscillator at the downstream end portion thereof and in DC. isolation with respect to the circuit, said probe being made of a refractory high tensile strength refractory metal such as tungsten, tantalum, rhenium, molybdenum or alloys thereof.

Other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings.

FIG. 1 is a longitudinal cross-sectional view partly in elevation of a microwave traveling wave electron discharge device of the backward wave oscillator type incorporating the teachings of the present invention.

FIG. 2 is a cross-sectional view taken along the lines 2-2 of FIG. 1 depicting the supporting arrangement for the slow wave circuit.

FIG. 3 is an illustrative graphical portrayal of a suitable tapered magnetic focusing field advantageously employed in the present invention.

FIG. 4 is an illustrative graphical portrayal depicting potential inside the beam from the upstream to the downstream end portions of a prior art backward wave oscillator such as schematically represented in. FIG. 4a which employs a positive ion draining probe disposed externally of the circuit.

FIG. 40! is a schematic representation of a typical prior art ion draining probe scheme utilized in a backward wave oscillator.

FIG. 5 is an illustrative graphical portrayal of potential inside the beam between the downstream. and upstream end portions of a backward wave oscillator incorporating the novel positive ion draining probe techniques of the present invention.

FIG. 5a is a schematical representation of the novel positive ion draining probe utilized in a backward wave oscillator according to the teachings of the present invention.

Referring now to FIG. 1 there is depicted therein a microwave traveling wave electron discharge device of the backward wave oscillator type 6. Since the prior art is replete with theoretical and experimental studies of the operations of microwave traveling Wave electron discharge devices of the backward wave oscillator type, a comprehensive explanation will not be repeated herein. See for example, an article entitled Backward Wave Oscillators by H. R. Johnson, Proceedings of the I.R.E., vol. 43, No. 6, June 1955, pages 684697. The backward Wave oscillator 6 depicted in FIG. 1 incorporates a suitable conventional electron gun or beam forming and projecting means 7 disposed at the upstream end portion thereof. The beam forming and projecting means 7 is adapted and arranged to produce a hollow electron beam. See for example, the aforementioned US. Patent No. 2,991,391 for the details of such a conventional hollow beam forming and projecting means 7. At the downstream end portion of the backward wave oscillator 6, the novel positive ion draining probe portion of the present invention is shown in detail. Disposed intermediate the downstream and upstream end portions of the backward wave oscillator 6 is a helical slow wave circuit 8 preferably of any suitable material such as for example, tungsten, supported along the axial extent thereof by preferably three dielectric rods 9 made of such dielectric materials as for example, sapphire. See FIG. 2 for more details. The helix 8 is preferably glazed to the sapphire rods and the resulting assembly is fixedly secured within the main body 10 of the backward wave oscillator 6. A suitable coaxial output arrangement 11 is connected at the upstream end portion of the helix for extracting the electromagnetic wave energy generated by the oscillator 6.

Preferably, a magnetic focusing scheme as disclosed in US. patent application Ser. No. 409,521 by Richard H. Ohtomo is utilized to provide a tapered magnetic field intensity between the upstream and downstream portions of the oscillator 6 in order to obtain suitable beam current interception along the axial extent of the slow wave circuit helix 8. FIG. 3 depicts an illustrative portrayal of magnetic field intensity H tapered along the axial extent of the tube to obtain the proper amount of beam interception along the circuit. Since the particular focusing scheme utilized in the present invention to provide beam interception along the axial extent of the helix does not form part of the present invention only fragmentary portions of same are shown, such as pole caps 13, 14.

At the downstream end portion of the backward wave oscillator 6 there is depicted an elongated positive ion draining probe 15 disposed along the central beam axis Z and disposed partially co-extensive with and internally of the slow Wave circuit 8, as shown. The probe is preferably made of such high tensile strength and high melting point materials as tungsten, tantalum, molybdenum, rhenium and alloys thereof and rigidly supported by a steel or the like ring support member 16 which is suitably brazed to a steel or the like support ring 17, as shown. A matching support ring 27 serves to provide a rigid support between cup-shaped support member 18 and the tube main body 10. A pair of cup-shaped support members 18, 19 preferably of a material such as Kovar support a ceramic insulation ring 20, such as of alumina, sandwiched therebetween which functions to provide D.C. isolation between the tube main body and helix and the probe assembly portion. A copper pinch off tubulation 26 is brazed to the steel support ring 16 and member 17 to complete the vacuum seal for the tube. Any suitable biasing scheme such as variable D.C. (direct current) supply battery 21 can be used to provide a suitable potential difference between the circuit helix 8 and the positive ion draining probe 15. The circuit 8 is preferably D.C. shorted to the main tube body 16 by means of conductive tab 22. Any suitable power supply such as, for example, variable D.C. supply battery 23 may be utilized to provide variable potential between the circuit 8 and the cathode lead 24. Alternatively, the cathode lead and ion draining probe 15 may advantageously be directly electrically tied together and the separate D.C. supply 21 eliminated. Copper pinch off tubulation 26 is of course, electrically tied to the probe 15 through the intermediary conductive support ring 16.

The graphical portrayal of FIG. 4 depicts the potential inside the hollow electron beam between the upstream and downstream end portions along the central axis Z of a backward wave oscillator incorporating a positive ion draining probe disposed externally of the slow wave circuit such as schematically represented in FIG. 4a. The lines labeled A, B, C, and D in FIGS. 1, 4-, 4a, 5, and 5a are representative of transverse planes taken through the cathode, anode, beam termination collection point at the downstream portion of the oscillator and the downstream end termination of the slow wave circuit, respectively. The curve labeled P is representative of the potential within the hollow beam between the cathode plane A of the beam forming and projecting apparatus 7 to the probe at the downstream end portion of the tube. It is noted that the potential rises between the cathode and the anode plane B which is circuit potential and thence proceeds to drop off due to space charge effects along the circuit length until the beam is totally dissipated at the downstream end portion of the circuit at the beam termination collection point C where the potential once again rises to circuit potential. The potential inside the beam and along the axis Z will then drop off to probe potential. It is thus seen that the probe is ineffective as a positive ion draining mechanism in the prior art scheme depicted in FIG. 4 and FIG. 4a. This is due to the fact that the circuit portion between the probe and the beam collection point acts as a highly effective shield thus eliminating or minimizing any effect of the probe with regard to eliminating the potential trap (darkened area) along the circuit.

FIGS. 5 and 5a depict the positive ion draining probe of the present invention disposed partially co-extensive with and internally of the slow wave circuit and as seen in FIG. 5 the potential trap has been effectively removed. Experimental results have indicated that probe potentials greater than 45 volts negative with respect to the helix or circuit potential are effective for providing the necessary elimination of potential traps along the extent of the circuit. Examination of FIG. 5a and the dashed equipotential lines emanating between the probe and the helix shOW that the shielding of the last few turns of the helix has been effectively eliminated by positioning of the probe partially coextensive with and internally of the helix itself. Experimental results have indicated that better than a 30 db improvement in spurious noise output and in PM noise output has resulted from the utlization of the probe arrangement depicted by the present invention as shown in FIG. 1 and schematically in FIG. 5a in comparison to an external probe arrangement as shown in FIG. 4a of the prior art. Although a tapered magnetic field such as shown in FIG. 3 is preferred in order to provide adequate beam interception along the axial extent of the slow wave circuit, it is to be understood that any other suitable technique may be employed to allow the beam to be totally or partially dissipated on the helix. Experimental results have indicated that the probe may be advantageously operated anywhere from cathode potential and lower to 45 volts negative with respect to the circuit potential without losing the positive ion draining effectiveness thereof.

Since many changes could be made in the above construction and since many apparently widely ditferent embodiments of this invention would be made Without departing from the scope thereof, all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in limiting sense.

What is claimed is:

1. A linear beam microwave traveling wave electron discharge device of the backward wave oscillator type defining a linear central beam axis with upstream and downstream portions including beam forming and projecting means disposed at the upstream portion and a slow wave circuit disposed along the beam axis, and a posi tive ion draining probe disposed at the downstream portion of said slow wave circuit and disposed only partially coextensive with and internally of said slow wave circuit, said ion draining probe being disposed in said oscillator in direct current isolation with respect to said slow wave circuit such that said probe may be biased negatively with respect to said slow Wave circuit whereby positive ions may be drained along the axial extent of said slow wave circuit in a manner such that positive ion drainage occurs in an axial direction towards said probe, said probe having its end portion terminated interiorly of said slow wave circuit at said downstream end portion, said device including focusing means for permitting said beam to be defocused along the axial extent of said device such that said beam is intercepted by said slow wave circuit along the axis of said device.

2. The backward wave oscillator defined in claim 1 wherein said slow Wave circuit is a helix and said probe extends therein at least two periodic lengths thereof.

3. The backward wave oscillator defined in claim 1 wherein said probe is direct current connected to the cathode portion of beam forming and projecting means disposed at the upstream end portion of said device.

4. The backward wave oscillator defined in claim 1 wherein said probe is adapted and arranged to be biased at least 45 volts negative with respect to said slow wave circuit.

5. The backward wave oscillator defined in claim 1 wherein said probe is made of a refractory metal or refractory metal alloy selected from the group consisting of tungsten, molybdenum, tantalum and rhenium.

6. A microwave linear beam backward wave oscillator including a hollow beam forming and projecting means disposed at the upstream end portion thereof and a slow wave circuit extending along the central axis thereof, and a positive ion draining probe disposed at the down stream end portion of said oscillator, said probe disposed partially co-extensive with and internally of said slow wave circuit, said probe being adapted and arranged to be biased negative with respect to said slow wave cir cuit, said probe being supported exteriorly of said slow wave circuit at the downstream end portion of said oscillator and protruding into the interior of said slow wave circuit from said downstream end and having its end portion terminating in the interior of said slow wave circuit at said downstream end portion thereof.

7. The backward wave oscillator defined in claim 6 wherein said probe is supported in direct current isolation with respect to said slow Wave circuit and terminates therein after extending along at least two periodic lengths of said circuit.

8. A microwave backward wave oscillator including a beam forming and projecting means disposed at the upstream end portion of said oscillator, a positive ion draining probe disposed at the downstream end portion of said oscillator, a helix slow wave circuit disposed therebetween, a metallic body surrounding said helix, said helix being shorter to said metallic body, said probe extending only-partially axially co-extensive of and interiorly of said slow wave circuit, said probe being located at the downstream end portion of said helix, said probe being supported in direct current isolation with respect to said helix, said beam forming and projecting means having the cathode portion thereof electrically tied to said probe.

References Cited UNITED STATES PATENTS 2,652,513 9/1953 Hollengerg 315-39.3 X

HERMAN KARL SAALBACH, Primary Examiner. ELI LIEBERMAN, Examiner. S. CHATMON, JR., Assistant Examiner. 

